1. Field of the Invention
The present invention relates to an image forming apparatus provided with a laser emitting section for emitting a laser beam.
2. Description of the Related Art
Various techniques for enhancing the accuracy of the scanning position of a laser beam have been conventionally devised for an image forming apparatus that deflects the laser beam modulated according to an image signal using a rotary polygon mirror and causes the laser beam to scan on a photosensitive drum. To enhance the accuracy of the scanning position, a technique for measuring a trail of the laser beam has also been proposed. This technique corrects the scanning position in real time using laser-detecting sensors arranged outside the opposite ends of the photosensitive drum. More specifically, optical sensors are used to align image formation start positions in the main scanning direction to thereby adjust image formation start timing of the laser beam in the main scanning direction. Also disclosed is that optical sensors are provided so as to measure intervals of time at which the laser beam passes by the sensors, and a scanning density in the main scanning direction is corrected (e.g. Japanese Patent Laid-Open Publication No. 2003-21799).
Based on the intervals of time at which the laser beam is detected by the sensors disposed at the both ends of the photosensitive drum, it is possible to determine an average scanning speed of the laser beam between the sensors. The above-described technique can determine the average scanning speed of the laser beam for one scan in the main scanning direction, but for finer correction, e.g. correction of the jitter of a polygon mirror motor, it is required to measure the scanning speed of the laser beam in each of sections formed by dividing a length corresponding to one scan in the main scanning direction.
When the scanning speed of the laser beam in the main scanning direction is desired to be measured more finely, it is required to arrange several sensors in an image forming area in the main scanning direction. However, in a case where the sensors are thus arranged, if an image is being formed by scanning of the laser beam, portions of the image where the laser beam is blocked by the sensors are shaded, which makes it impossible to form a normal image.